As is well known to those skilled in the art, hydrocarbon stocks used for the preparation of lubricating oil contain undesirable components which are removed during processing to permit attainment of a product which is characterized by desired properties. Typically a charge stock is subjected inter alia to a dewaxing operation which decreases the wax content and desirably lowers the pour point and the cloud point of the lubricating oil stock.
Dewaxing may be effected by mixing a dewaxing solvent, such as methyl ethyl ketone or methyl isobutyl ketone, optionally in the presence of a hydrocarbon such as toluene or xylene, with the charge oil and cooling the mixture below the desired pour point, typically to minus 5.degree. F.-minus 20.degree. F., say minus 10.degree. F., at which point wax precipitates. The wax is filtered; and the dewaxed oil-dewaxing solvent mixture is recovered.
This recovered mixture has classically been flashed to recover solvent-free lubricating oil stock and solvent which is free of lubricating oil.
Those skilled in the art have attempted to improve the economics of this process by utilizing membrane technology to separate dewaxing solvent from dewaxed oil. Illustrative of such prior art attempts may be the following:
U.S. Pat. No. 4,985,138 to Pasternak discloses a cross-linked polyimine membrane.
European Patent Specification 0 220 753 Al to Bitter et al discloses use of a halogen-substituted silicone membrane.
U.S. Pat. No. 4,678,555 to Wernick discloses use of cellulose acetate membranes.
European Patent Specification 0 125 907 Al to Wight discloses use of a polyimide membrane containing -CO-NO-linkages.
U.S. Pat. No. 4,617,126 to Funk et al discloses use of a polysulfone membrane.
Additional background may be obtained from the references set forth in these patents.
It is an object of this invention to provide a process for treating a charge containing dewaxing solvent and dewaxed oil. Other objects will be apparent to those skilled in the art.